The present invention relates to a machine for silk-screen printing on a flat surface, such as for instance a sheet of paper or similar material, or flat surfaces realised with other materials, such as the flat surfaces of printed circuit boards for instance.
The known type of machines for screen-printing on flat surfaces generally comprise a fixed structure to which is linked in a mobile way a first frame connected to a support plane on which the sheet to be printed is positioned by means of extraction from a feeding station. Also linked to the fixed structure are a second frame which supports a printing screen, and the members to guide and move a printing bridge onto which a doctor is attached.
The printing cycle involves the separation of the support plane from the structure supporting the screen and the printing bridge during the phase of admission of a new sheet to be printed, to allow the means for positioning the sheet to pass between the underlying support plane and the overlying frame that supports the screen and the printing bridge.
As well as machines in which the movement between these two frames is effected xe2x80x9clike an opening book,xe2x80x9d (with the frames hinged in correspondence to the exit side of the printed sheet), machines are known for screen-printing on flat surfaces in which the movement between the two frames occurs along a direction perpendicular to the support plane of the surface to be printed.
To carry out such movements, it is known to use sliders hat run along suitably shaped guides, or cam disks of the xe2x80x9ccam nosexe2x80x9d type are used to impart the desired movement during each cycle of printing. The principal drawbacks of the known machines is due in particular to the fact that the systems adopted for the movement of the frames are particularly complex and subject to wear. For this reason the frequent adjustment operations, necessary to compensate for the wear to which the parts are subject, require particularly long intervention time and machine downtimes.
A purpose of the present invention is to propose a machine for screen-printing on a flat surface that considerably reduces the adjusting interventions to maintain the synchronisation between the various mobile parts.
Another purpose of the present invention is to propose a machine of the type mentioned above that particularly simplifies interventions for maintenance and for adjusting the mobile parts.
A further purpose of the present invention is to propose a simple machine for screen-printing on flat surfaces in which the movement of the frames can be achieved at particularly reduced cost.
These purposes are achieved by the present invention, which relates to a machine for screen-printing on a flat surface, of the type comprising a fixed structure, at least a first mobile frame linked to a support plane for the sheet to be printed, and at least a second mobile frame which carries a printing screen and the members to guide a move a doctor, characterised by comprising means for cyclically moving the first and second frames in a direction perpendicular to the support plane between an open position, in which the sheet just printed is removed and a blank to be printed is inserted, and a closed position in which the sheet to be printed is maintained on the support plane during the printing phase, characterised in that the means for cyclically moving the first and second frames comprise a plurality of pairs of eccentric wheels rotatably fitted to the fixed structure and respective follower rollers fitted to the first and second frames. The use of eccentric wheels instead of the traditional cams gives a more regular motion to the frames and limits the inertia which they develop, even at particularly high working speeds.
Furthermore, eccentric wheels are simpler and undoubtedly less expensive to be produced than the cams used in the machines of the known type.
Each pair of eccentric wheels is driven by a common shaft with one wheel out of phase with respect to the other by around 180xc2x0. Each pair of eccentric wheels comprises a first wheel acting on at least one follower roller fixed to the first frame and a second wheel acting on at least one follower roller fixed to the second frame.
The aforementioned frames are substantially rectangular in plan. The first frame preferably is slightly smaller than the second frame and is mobile inside the space delimited by the second frame. Four pairs of eccentric wheels are therefore located substantially in correspondence of the angles of the first and of the second frame to get a particularly precise and balanced movement of the frames.
According to a particularly advantageous aspect, the eccentric wheels are rotated by a common motor through transmission driving means. This motor is separate from the motors that operate the other mobile members in such a way as to allow the application of computerised numerical control devices to synchronise the movements of the two frames with respect to other mobile members of the machine, such as for instance the doctor and the means for positioning the sheets with respect to the support plane. Furthermore, the means for transmission of the motion between the motor for the movement of the frames comprise a plurality of shafts and one or more drive distribution boxes. This allows the drive system of the eccentric wheels to be installed at particularly reduced costs in printing machines of different dimensions. In fact, it is sufficient only to vary the length of the motion transmission shafts, while maintaining unchanged the drive distribution boxes and the bearing fixing the pairs of eccentric wheels to the fixed structure of the machine.
To get a high precision movement of the frames with respect to fixed structure, a plurality of vertical guide elements, integral to the fixed structure, are preferably provided.